Originally Posted by Senrab62

They can be bad for knees. I've had both of my ACLs reconstructed, with surgeries dating back over 20yrs.

With proper fit, appropriate gearing (spin to win!) and proper brakes I don't have any excessive pain or even pain at all.

Modulating speed by skidding and gearing is when I have pain. Otherwise, it's no different than riding a geared bike for me. It's why I never understood huge ratios and no brakes.